[ads] New Tab Takeover video ad design fixes

[aseren]

• Change size of NTT video ad play button to fit the design
• Change gradient to be on top of the NTT video ad
• If a user exits full screen, the video should return to the keyframe in the background
• After the video plays in full, the video should return to the keyframe in the background

[LaurenWags]

Requires 1.66.95 or higher to test

[hffvld]

Verified on iPhone 14 and iPad Air using version(s):

Device/OS: 
- iPhone 14 / iOS 17.5
- iPad Air / iPadOS 16.7.2
Brave build: 1.66 (95)
BraveCore: 1.66.95 (124.0.6367.91)

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STEPS:

1. Follow the steps from [ads] New Tab Takeover video ads design fixes brave-core#23149 (comment)
2. Verify

ACTUAL RESULTS:

• Verified that NTT video ad autoplay is started when NTP with New Tab Takeover video ad is opened
• Verified that the Play button becomes visible when the NTT video ad autoplay is finished
• Verified that stop keyframe is shown after NTT video playback is finished (The stop keyframe is the frame where autoplay finishes)
• Verified that stop keyframe is shown after the user cancels the playback (The stop keyframe is the frame where autoplay finishes)

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iPhone

2024-04-30_10-16-38.mp4

iPad

2024-04-30_10-39-51.mp4